Diesel engines detect a NOx (nitrogen oxide) concentration in exhaust gas with a NOx sensor that is disposed on an exhaust passage to purify NOx in the exhaust gas, and perform adjustment of an injection quantity of urea water and self-diagnosis of a catalyst or the like based on the detected value.
As a device for diagnosing deterioration or malfunction of the NOx sensor, a device for diagnosing deterioration or malfunction of a NOx sensor by comparing NOx concentrations before and behind a reduction catalyst, after supply of a reductant used for the reduction catalyst is stopped is proposed (For example, see Patent Literature 1).
This device diagnoses the deterioration or the malfunction of the NOx sensor during traveling of a vehicle. However, in the diagnosis during the traveling of the vehicle, even when the supply of the reductant is stopped, there is an influence of a reduction action of the reduction catalyst due to a reductant remaining in the exhaust passage or a reductant adsorbed on the reduction catalyst, and it is difficult for the NOx concentrations before and behind the reduction catalyst to be approximately matched with each other. For this reason, in the malfunction diagnosis of the NOx sensor in the device, the diagnosis cannot be performed with high accuracy, and there is a concern that the NOx sensor subjected to neither deterioration nor malfunction is replaced by mistake.